The device concepts are based on the use of microresonator rings as the filter elements within a laser cavity, either as part of a reflector in a linear laser cavity, or part of the filter within a ring laser cavity. These laser cavity designs have been used previously with optical microresonator rings forming the filters/reflectors; in these previous cases the devices used a maximum of two microresonators with different ring radii, placed in series to provide the filter function, this filter function having limitations leading to reduced laser performance; relatively large linewidth, low output power, poor relative intensity noise (RIN), and limited mode selectivity.
Previous tunable laser designs based on ring reflectors in a linear cavity, using a III-V monolithic semiconductor platform such as “Full C-Band Tuning Operation of Semiconductor Double-Ring Resonator-Coupled Laser With Low Tuning Current” by T. Segawa et al, IEEE PHOTONICS TECHNOLOGY LETTERS, 19, pages 1322-1324, 2007, and “Microring-Resonator-Based Widely Tunable Lasers”, by S. Matsuo et al, IEEE JOURNAL of SELECTED TOPICS in QUANTUM ELECTRONICS, 15, pages 545 to 554, 2009, or using a silicon photonics platform such as “Compact, lower-power-consumption wavelength tunable laser fabricated with silicon photonic-wire waveguide micro-ring resonators”, by T. Chu T et al, OPTICS EXPRESS, 17, pages 14063 to 14068, 2009, and “25 kHz Narrow Spectral Bandwidth of a Wavelength Tunable Diode Laser with a Short Waveguide-Based External Cavity”, by R. M. Oldenbeuving et al, LASER PHYSICS LETTERS, 10, 015804, 2013, utilized a reflector incorporating two rings with slightly different radius, and therefore different Free Spectral Range (FSR). These two rings are tuned using a Vernier approach; one resonance from each ring is aligned to provide a small pass band through the combined filter, all other wavelengths within the gain bandwidth of the gain element being blocked. A ring-cavity laser, also using two rings and the Vernier effect for tuning, described in “Widely tunable vernier ring laser on hybrid silicon”, by J. C. Kulme et al, OPTICS EXPRESS, 21, pages 19718 to 19722, 2013, was fabricated using heterogeneous integration on a silicon photonics platform. A schematic and picture of the monolithically integrated III-V laser by Matsuo is shown in FIG. 1, with the version more recently developed using silicon nitride (Si3N4) waveguides on a silicon on insulator (SOI) substrate and an external gain chip, by Oldenbeuving, shown in FIG. 2(a); the power reflectivity of the reflector in this device is shown in FIG. 2(b).
The III-V based device in FIG. 1 used the facets on both sides of the laser for reflectors, with the two rings filtering the signal passing though them in a double pass scheme. The device in FIG. 2 (a) had an external gain chip, and achieved reasonable results; C-Band tunability, <25 kHz linewidth, and 50 dB Side Mode Suppression Ratio (SMSR), however, the output power of the laser was very low, only 1 mW. The 50 dB SMSR, while typical for this and other reported devices, and for distributed feedback (DFB) lasers, indicates too high a value for the RIN of the device for use in RF photonics and other high performance applications, as the RIN of a laser is directly proportional to its SMSR value. An SMSR of ˜70 dB can be seen in lasers with very low RIN. The designed reflector response for the device in FIG. 2(a) is shown in FIG. 2(b); this reflector has insufficient suppression of reflections from the non-lasing cavity modes to obtain ultra-low noise operation.
One group of previous works utilized two small waveguide based rings with different FSR, e.g. III-V or silicon microresonators, to provide the correct mode selectivity for singlemode lasing; the smaller the rings the higher the FSR, and the higher overall mode selectivity. Tunable lasers that covered the C-Band (1535-1565 nm) were fabricated, however, the relatively high optical loss of silicon or III-V waveguides, especially when used in small microresonators (radii of ˜10 microns), e.g. 2 to 4 dB/cm for silicon, gave rise to lossy filters/reflectors, and therefore short effective cavity lengths—which do not provide the required narrow linewidth operation and high power needed for advanced systems. The small microresonators are also operated with high Q, providing very high power density within the rings—leading to self-heating (and changing the ring resonance frequency) and also nonlinear effects within the rings. This limits the possible power levels at which these devices can operate.
A lower loss waveguide/microresonator material, Si3N4 was used in prior art, where the dual ring microresonator based reflector was coupled to a semiconductor gain chip to form the external cavity laser (ECL) through hybrid integration. Reasonable results were found, but again, by using two rings and the Vernier effect in the reflector, using low but not ultra-low loss waveguides/microresonators, devices had limited mode selectivity, had low output power and relatively large linewidth.
There is a need for an increase in the effective cavity length of laser devices, while at the same time keeping optical losses low and mode selectivity high over a wide wavelength range, in order to overcome current limitations in laser devices which do not provide sufficiently narrow linewidth operation while also providing high output power and low RIN. There is a need for lasers with this high performance that can operate at a specific wavelength, or be able to be broadly tunable over a wide wavelength range.